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CarlH

Briggs Head Torque

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CarlH
For the second year in a row, the head gasket on the 16 hp Briggs in my 7016H blew the first time I mowed in the spring. Last year I replaced the gasket, retorqued to spec (190 in-lbs), mowed all season, and blew a little snow through the winter. After mowing about 45 minutes (just getting well warmed up), it blew this year. Since I'm not one that's much for coincidences, I examined EVERTHING carefully this time. I noticed some nicks in the gasket mounting surface on the head in the area of the break (the initial break was between the head and the gasket)and also a small gap on the outside of the mounting surface of the block in that area. The nicks were minor and not something that I would expect to be unsealable by the head gasket. I have replaced the head with another and (obviously) a new gasket. I am now wondering about the head bolt torque spec - 190 in-lbs (less than 16 ft-lbs)in the Briggs manual. According to the Simplicity standard torque chart, a 5/16-18 Grade 8 bolt should be torqued to 25 ft-lbs. Anybody have any insights? Thanks

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HubbardRA
I use an unconventional method of torqueing the heads on these small engines. I tighten all of the head bolts snugly by hand with a wrench, in the specified sequence. Then I take a medium size ball-peen hammer and tap the end of the wrench until the bolt stops turning. I have never broken a head bolt and have not had a head gasket blow with this method. I know this is an off-the-wall method, but it has worked for me for years.

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UCD
Carl You can repair thos nicks if you feel that they might be causing your problem. Get some JB Weld and mix it up and useing a a plastic spreader smooth it in to the nicks leaveing it slightly high. When dry sand it down level with the head. When torqueing the bolts make sure the thread are clean on the head bolts and clean the bolt holes with a tap and blow them out. Lightly oil the bolt threads before installing them. torque in 2 steps 12ft.lbs. then 16ft.lbs. Tap the head bolts with a hammer to remove preload between steps. The simplicity chart is a rule of thumb for most bolts but the spec of MFG should be followed Briggs specs are probably taking into consideration heat expansion. 190 in.lbs. = 190 ÷ 12 = 15.83333 rounded to 16 ft.lbs.

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CarlH
Maynard, This time I replaced the head with another I had which had no nicks and that I checked for being warped. I thoroughly cleaned the bolt threads, ran a bottoming tap into the bolt holes in the block, and hand threaded each bolt into its hole to assure that the threads were OK (both last year and again last night). I also torqued in increments but larger than 2 ft-lbs.

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CarlH
jj I have a click type Craftsman torque wrench that I bought new and it has not been abused although you can never really know if it was calibrated accurately in the first place. John, Who knows what has happened before I got this tractor 5-6 years ago. I wouldn't think that at these low torque levels there would be much stretching but I suppose someone could have used the 1/2" breaker bar on them before I got it. I would think that severe over-torqueing would result in a less elastic bolt and/or distortion of the aluminum head. I see no distortion of the bolt threads on any of the bolts.

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Leroy
You won't necessarily see see bolt distortion When a bolt reaches it's elastic limit it will twist off inside the head like a piece of string cheese. If you were to follow Maynards advice the problem would go away. Try it now or try it when it blows again, his advice here is not a guess, It works.

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Salthart
If I had that trouble 2 years running you can bet I'd pull the engine and check for a rats nest etc. Other things I would look for would be the location of the bolt where the gasket blows and its relation to the exhaust valve. If you have or know someplace where you can get hold of one try a temp gun and use it while mowing. Things like slow fuel flow can cause a lean burn under some conditions while running seemily normal. Low float level can do the same. Having pulled the head twice now I would figure you removed and carbon build up and so we can maybe skip pre-detonation due to carbon. ( remember, there are quite a few more square inches surface area on the head than the piston so the head takes a greater hit than the piston ) Even point gap can cause a cumulative heat/stress problem. Not that I'm saying any of the above IS your trouble.. Just things I would look for.. Good luck.

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CarlH
Marty, I checked for junk in the fins. The break area is not in the valve area (although in the same area as the previous one). This time I used different head bolts from another engine in this area. I also cleaned the carbon accumulation both times. Not excessive last year and less this year. Roy, I used a Briggs gasket.

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Leroy
Right out of my Briggs & Stratton service and repair manual June of 1981. Section 6 Compression page 1 Always note the position of the different cylinder head screws so that they may be properly reassembled. If a screw is used in th ewrong position it may be to short and not engage enough threads. It may be to long and bottom on a fin either braking the fin or leaving the cylinder head loose. ----------------------- ME. If you were to buy new head bolts the probability is that the dealer would share with you which bolt size goes in which hole if you ask. This activity would remove two potential variables. stretched or necked bolts and bolt position. The geometric flatness of the sealing surface should be determind. this can be achived by rubbing black magic maker on the mating surfaces and taking the ruler from an adjustable square and lightly skim a layer of air off of the mating surfaces. The high areas will scrape the low areas will not. this will tell you some flat work will be required and it is best left to those who have a knack for it.

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CarlH
I'm running fine for now. I think that Marty has several good things to look into although the engine doesn't seem to be running hot even in heavy wet grass. I'll get my digital thermometer out one of these days and see if the head has any hot spots. Leroy, I was careful about getting the head bolts in the right holes and the bolts I stole from my other 16 HP Briggs were from the same holes. None of the bolts show any distortion of any kind. I am thinking it was the old head being slightly warped. If the gasket goes again, I'll probably get new bolts and another head as well as surfacing the block surface.

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